nurl/models/url.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const CounterSchema = Schema({
    _id: {type: String, required: true},
    seq: {type: Number, default: 1000}
});

const counter = mongoose.model('counter', CounterSchema);

// create a schema for our links
const urlSchema = new Schema({
    _id: {type: Number, index: true},
    long_url: String,
    created_at: Date,
    visits: {type: Number, default: 0}
});

urlSchema.pre('save', function(next){
    const doc = this;
    counter.findByIdAndUpdate({_id: 'url_count'}, {$inc: {seq: 1} }, function(error, counter) {
      if (error)
          return next(error);
      doc.created_at = new Date();
      doc._id = counter.seq;
      next();
  });
});

const Url = mongoose.model('Url', urlSchema);

module.exports = Url;